The body has a number of mechanisms by which to rid itself of undesirable cells, and many diseases result from defects or deficiencies in these processes. For example, although the immune system is believed to play an important role in preventing the development of tumors or curtailing their growth or spread, it is clear that in many cases immune surveillance is not sufficient to prevent cancer. Particular populations of cells can also cause or contribute to a variety of other diseases. For example, white blood cells of various types may cause rejection of a transplanted organ, auto-immune disease, or allergies. Proliferation of vascular tissue in the eye can lead to vision loss. Molecules and other entities that are not properly removed or disposed of can also cause disease. Examples of such entities are calcium, fatty acids, lipids, uric acid, bilirubin, etc. Thus it is evident that the body's own mechanisms for detecting and removing cells and harmful noncellular molecular entities are frequently insufficient. There is a need for improved methods to remove such undesirable cells or noncellular molecular entities.
Despite significant advances, cancer continues to be an extremely difficult disease to treat, claiming ˜550,000 lives annually in the United States alone (49). In addition, many currently available pharmaceutical agents target dividing cells generally rather than being selective for cancer cells. This feature is responsible for numerous side effects that frequently limit the acceptable dose that can be delivered to a patient. New therapeutic strategies are urgently needed. In particular, there is a need for therapeutic strategies that would more specifically target cancerous cells. In addition, there is a need for strategies that would synergize with or enhance existing methods of treating cancer.